Side airbag apparatuses are a type of airbag apparatus in which a folded side airbag is accommodated in a seat back of a vehicle, and which cause the side airbag to expand between a vehicle occupant and a side wall of a vehicle body to protect the vehicle occupant when a side part of the vehicle receives an impact. This type of side airbag apparatus is disclosed in Japanese Patent Laid-Open Publication No. 2005-313688. This side airbag apparatus shall be described below with reference to FIG. 5 hereof.
In a side airbag apparatus 100 shown in FIG. 5, a side airbag 101 is divided by a dividing wall 102, and comprises a chest region protecting chamber (upper expansion chamber) 103 and lumbar region protecting chamber (lower expansion chamber) 104.
The side airbag 101 is caused to expand by gas discharged from an inflator 105, with the gas supplied to the upper expansion chamber 103 and lower expansion chamber 104 as indicated by the arrows. The upper expansion chamber 103 expands toward a chest region 106a of a vehicle occupant 106. The lower expansion chamber 104 expands toward a lumbar region 106b of the vehicle occupant 106. The chest region 106a and lumbar region 106b of the vehicle occupant 106 are supported by the upper expansion chamber 103 and the lower expansion chamber 104, respectively.
When gas is supplied from the inflator 105 to the lower expansion chamber 104, the gas flows toward a front part 104b as indicated by the arrow a along an inner surface of a lower part 104a of the lower expansion chamber 104, and flows upward in the front part 104b, whereby an upward pressing force Fa presses upward on the front part 104b. This upward pressing force Fa causes the lower expansion chamber 104 to move upward. For this reason, the lower expansion chamber 104 does not readily expand in a stable manner, and considerable scope exists for improvement.